Smoke exhaust fans are used in emergency exhaust ventilation systems for forced extraction of smoke and heated gases and simultaneous transfer of heat away and beyond the limits of the serviced spaces in case of fire. Such units are used in production, public, residential, administrative and other spaces.

OPERATION

The fans are capable of handling smoke and air mixtures with temperatures up to +600 °C for 120 minutes. The fans can operate in conjunction with a frequency converter (hereinafter referred to as FC) or directly when connected to power supply. The corresponding characteristics for these operating modes (operate from mains and/or frequency converters) for each fan are given in the brochure, in the Downloads section.

The fan can be used for general exhaust ventilation at a rotational speed reduced by at least 25 % of the rated speed of the electric motor. The fan can be designed for moderate (U) or tropical (T) climate conditions.

DESIGN

The fans are made of heat resistant steel with polymer coating providing weathering resistance. There are two types of roof-mounted smoke exhaust fans: with horizontal air discharge (VKDH) and vertical air discharge (VKDV). The models with vertical air discharge are equipped with a backdraft damper. The fan has a protective grille to prevent accidental contact and penetration of foreign objects. The impeller with backward curved blades are painted with powder coating.

MOTOR

The fans are equipped with three-phase electric motors rated for 400 V. The motor is located in the compartment offset from the transported air stream.

Motor ingress protection rating is IP54.

INSTALLATION

The roof fans are installed on the RKV mounting frame (simplified version) or RKVI mounting frame (heat-insulated version). The RKV and RKVI mounting frames are designed for mounting the fan on the roof without a slope. Make sure that the location provides for sufficient space as required for the fan maintenance.

Ventilation systems produce a heavy load on the electrical systems of a building.

Therefore, reducing the power consumption is among the top priorities for construction project designers.

DIRECT ON-LINE STARTING (DOL)

FREQUENCY CONVERTER

As a rule, smoke extraction systems utilize large high-performance fans. During starting the high inertia of the shaft causes a substantial increase in the start-up time - i.e. the time from the application of power to reaching the rated speed. As a result the motor is subjected to high starting current for a prolonged period of time.

Current diagram with direct on-line starting

Standard switchgear (automatic circuit breakers, contactors and motor starters) is not designed to withstand prolonged overloads causing the fan to shut down automatically during starting.

Using switchgear with a higher maximum current rating renders the electric motor protection system less sensitive.

As a result the switchgear will not be able to detect motor overload in time due to a higher current sensing threshold.

Such challenges can only be addressed by utilizing a soft starter or a frequency converter to start the fan without causing a prolonged overload.

Torque diagram with direct on-line starting

A frequency converter (FC) consists of two main component blocks. One converts alternating current (50 or 60 Hz) into direct current.

The other converts direct current into alternating current of variable frequency ranging from 0 to 250 Hz.

By controlling the frequency output the FC offers a broad range of motor speed regulation.

Current diagram with FC starting

During the starting the FC raises the frequency from 0 Hz to the electrical mains frequency (50 or 60 Hz).

As the frequency is increased gradually, the motor can be assumed to operate at its nominal speed for a given frequency value.

Furthermore, on the assumption that the motor runs at its nominal speed the nominal torque should be immediately available whereas the current will be approximately equal to the nominal value.

Torque diagram with FC starting

SOFT STARTER

APPLICATIONS IN VENTILATION SYSTEMS

Unlike an FC, a soft starter does not change the frequency of speed.

Instead it gradually increases the voltage supplied to the motor - from the initial level to the nominal level.

Current diagram with soft starter

Torque diagram with soft starter

The very low voltage initially supplied to the motor during starting helps avoid sudden jolts.

Both voltage and torque increase gradually.

Using an FC or a soft starter allows to reduce the starting current thereby avoiding a loss of voltage in the electrical mains.

In addition to that the starting torque and mechanical strain on equipment are also reduced which translates into extended maintenance and repair intervals.

General exhaust ventilation (GV) systems and emergency smoke extraction (SE) systems of car parks, warehouses, utility and industrial spaces can be integrated into a single system at the design level.

As the air flow rate a GV system is considerably lower than that in an SE system a high-capacity fan rated for smoke extraction duty runs in partial load mode.

Frequency converters can be programmed for several fixed-speed settings to enable operation in GV and SE modes.

The fan operating modes can be switched upon receiving an external signal from the control system.

Such integration of ventilation systems allows overall cost savings.

FC-51 AND FC-101: WHEN TO USE?

Below you can find two series of frequency converters: FC-51 and FC-101.

The FC-51 series is only suitable for general ventilation systems.

If intended for a smoke extraction system, the fans should be fitted with the FC-101 series units as they feature an integrated fire mode.

When activated the integrated protection equipment of the frequency converter is overridden enabling the unit to continue operation despite the possibility of sustaining irreparable damage due to overheating or overloading.

In case of a fire, the FC-101 series frequency converters are capable of maintaining a higher air pressure level at landings compared to other areas of the building in order to keep the landings smoke-free.